MXPA02004558A

MXPA02004558A - Burner air fuel ratio regulation method and apparatus.

Info

Publication number: MXPA02004558A
Application number: MXPA02004558A
Authority: MX
Inventors: G Tesar Michel
Original assignee: Megtec Sys Inc
Priority date: 1999-11-09
Filing date: 2000-10-17
Publication date: 2002-10-23
Also published as: JP2003514212A; JP5025060B2; US6213758B1; CA2389825C; EP1230517A4; EP1230517B1; AU1966501A; WO2001035025A1; EP1230517A1; CZ305079B6; AU766640B2; CA2389825A1

Abstract

Control system and method for regulating the air fuel mix of a burner for a web dryer or a regenerative or recuperative oxidizer, for example. Differential air pressure is monitored between the air chamber (21) of the burner (10) and the enclosure (15) into which the burner (10) fires (such as a flotation dryer or the combustion chamber of a regenerative thermal oxidizer). Fuel flow is monitored by a differential pressure measurement between the fuel chamber (22) of the burner (10) and the enclosure (15) into which the burner (10) fires. These measurements are compared to predetermined values, and the fuel flow and or air flow to the burner (10) is regulated accordingly.

Description

METHOD AND APPARATUS FOR THE REGULATION OF THE AIR / FUEL PROPORTION OF A BURNER The present invention describes burners, and more particularly to a method and apparatus for regulating the proportion of air and fuel in the burner to optimize the performance of the burner. BACKGROUND OF THE INVENTION In the drying of a moving web of material, such as paper, a film or other sheet material, it is sometimes desirable that the web be supported without contact during the drying operation, in order to avoid damage to the screen or to any inking or coating on the surface of the screen. A conventional arrangement for non-contacting and drying a moving web includes upper and lower sets of air bars that extend along a substantially horizontal elongation of the web. The hot air emitted by the air bars that support the weft in a floating manner and facilitate the drying of the weft. The arrangement of the air rods typically inside a dryer housing that can be maintained at a slightly sub-atmospheric pressure by means of a suction fan that extracts the volatiles that come from the weft as a result of drying the ink on that , by Ref. 138761 example. An example of a dryer can be found in U.S. Patent No. 5,207,008, of which the description is incorporated herein by reference. This patent discloses an air countercurrent separation dryer with an integrated back burner, wherein a plurality of air bars are placed above and below the path of the weft for non-contact drying of the coating in the weft. In particular, the air rods are in communication to receive the air with an elaborate support system, and blown air heated by the burner to the weft to support and dry the weft as it travels through the dryer housing. The regenerative thermal apparatus is generally used to incinerate the contaminated process gas. For this purpose, a gas such as contaminated air is first passed through a hot, heat exchange bed and into a high temperature communication oxidation chamber (combustion), and then through a second exchange bed. of heat relatively cold. The apparatus includes a number of internally isolated heat recovery columns containing heat exchange means, the columns are in communication with an internally isolated combustion chamber. The process gas is fed into the oxidant through an intake manifold containing a number of hydraulically or pneumatically operated flow control valves (such as spring valves). The process gas is then directed into the heat exchange media which contains "stored" heat from the previous recovery cycle. As a result, the process gas is heated close to the oxidation temperature by the media. Oxidation is completed as the flow passes through the combustion chamber, where one or more burners are located (preferably only to provide heat for the initial start-up of the operation in order to bring the temperature of the combustion chamber to the appropriate predetermined operating temperature). The process gas is maintained at the operating temperature for a sufficient amount of time to complete the destruction of the volatile components in the process gas. The heat released during the oxidation process acts as a fuel to reduce the burner output required. The process gas flows from the combustion chamber through another column containing the heat exchange media, thereby cooling the process gas and storing the gas therein in the media for use in a subsequent cycle of intake when the flow control valves are inverted. The resulting clean process gas is directed through an outlet valve through an outlet manifold and released into the atmosphere, generally at a temperature slightly higher than that of the admission, or recirculated back to the admission of the oxidant. According to the science of conventional combustion, each type of burner flame (eg pre-mixed flame, diffusion flame, vortex flame, etc.) burners with an optimum burner pressure ratio different from that of the fuel with the combustion air, for a given ignition rate, by means of which optimum stoichiometric low emission concentrations appear in the combustion gas of the burner. Therefore, it is important to control or maintain the desired optimum burner air / fuel pressure ratios of the burner. A failure to regulate with great precision the air / fuel ratio of the burner over the range of the burner outlet can lead to poor quality and flame stability (accidental extinction of the flame due to fuel shortages, yellow flames, etc.) or due to excessive contamination (N0X high, CO). For this purpose, U.S. Patent No. 4,645,450 describes a flow control system for controlling the air and fuel flow of a burner. The differential pressure sensors are placed in the air flow and the gas flow conduits feed the burner. Optimal differential pressures of air and fuel flow are determined through experimentation and analysis of combustion gas and storage in a microprocessor. These optimum values are compared to measure the values during the operation, and the flow of air and / or fuel to the burner is regulated based on the comparison when opening or closing the respective valve. This system does not detect the back pressure in the burner. It also generates a "signal" of fuel flow indicative of the proportion of fuel in the burner rather than through the burner. The mechanical valves used in conventional systems are connected by adjustable cams and connections to control the volumetric flow rates of air and fuel. However, if the air density changes due to atmospheric pressure and / or temperature variations, the proportion of air to the fuel is disturbed. In addition, mechanical valves are subject to wear and to the connection of cams and connections over time, and considerable skill is required to adjust the device. Systems that use mass flow measurement devices have a prohibitive cost. It is therefore an object of the present invention to optimize the mixing of fuel and air in a burner over a range of firing rates. It is therefore a further object of the present invention to provide a control system for a burner and thereby increase the efficiency of the burner. It is another object of the present invention to reduce gas emissions from the combustion of a burner. SUMMARY OF THE INVENTION The problems of the prior art have been overcome by means of the present invention, which provides a control system and a method for regulating the air / fuel mixture of a burner for a weft dryer or a regenerative oxidant or recuperative, for example. The differential air pressure is observed between the burner's air chamber and the housing where the burner ignites (such as a flotation dryer or the combustion chamber of a regenerative thermal oxidizer). The fuel flow is observed by means of a measurement of the differential pressure between the fuel chamber of the burner and the housing where the burner ignites. These measurements are compared for the predetermined valves, and the fuel flow and / or the air flow to the burner is regulated accordingly. The regulation of the air flow is achieved with a fuel blower with a motor controlled by variable speed drive that has both acceleration controls and deceleration controls, more than with a damper to achieve a more accurate and faster burner modulation and to use less electrical energy. In addition, the preferred drive can incorporate dynamic braking technology for more precise control. Dynamic braking is desired for rapid dissipation of voltages from high DC collective lines that are generated when the motor rapidly decelerates. The excess voltage is applied to the braking resistors, allowing the motor to decelerate faster. The present invention uses the burner housing to provide a direct measurement of the proportions of air and fuel flow, thereby eliminating costly flow measurement devices.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of the burner of the present invention shown mounted in a housing; Figure 2 is a graph of a supplier supplying the air and fuel settings for a burner; Figure 3 is a schematic view of the control system according to the present invention; Figure 4 is a graph showing N0X emissions from a burner in various fuel / air ratios; Figure 5 is a graph showing methane emissions from a burner in various fuel / air ratios; Figure 6 is a graph showing carbon monoxide emissions from a burner in various fuel / air ratios; Figure 7 is a graph comparing the current air pressure with the desired reference point over the opening range of the total valve; and Figure 8 is a graph comparing the current fuel pressure with the desired reference point over the opening range of the total valve.

DETAILED DESCRIPTION OF THE INVENTION First with reference to Figure 1, there is shown generally 10 a burner having a fuel inlet 12 and an air inlet 14. These admissions are connected to the fuel and air fuel sources, respectively, by means of respective suitable conduits, for example, any suitable fuel can be used as the fuel source of the burner, such as natural gas, propane, and fuel oil. The preferred fuel is natural gas. The burner is shown mounted in a housing or in the chamber 15. In one application of the present invention, the housing 15 is the housing of an air countercurrent separation screen dryer. In another application of the present invention, the housing 15 is the combustion chamber of a regenerative thermal oxidizer. The previous examples of the housing 15 are exemplary only; those of skill in the art will appreciate that the present invention has applications beyond those illustrated. A pressure opening 17 is shown in the housing, providing a site for differential charging of the air pressure and fuel sensors as described below. This opening can be located near the burner to provide a rapid response to changes in housing pressure. Typically, this opening 17 can be found 30.48 cm (12 inches) from the burner installation. The burner 10 includes a fuel pressure opening 18 and an air pressure opening 19 as shown. As is conventional in the art, the burner 10 includes an air chamber 21 and a fuel chamber 22. Referring now to Figure 3, the means for indicating the fuel flow and the air flow will now be described. The fuel differential pressure sensor 30 is shown in communication with the burner 10, and more specifically, in communication with the fuel chamber 22 of the burner 10. In addition, the fuel differential pressure sensor is in communication with the housing through of the pressure opening 17. The fuel differential pressure sensor 30 is also in communication with the controller 50, which generally includes a microprocessor that has a memory and preferably is a programmable logic controller (PLC). The fuel differential pressure sensor 30 detects the pressure differential between the burner chamber 22 of the burner 10 and the housing 15, and sends a signal indicative of this difference to the controller 50. The air differential pressure sensor 32 is shown in FIG. communication with the burner 10, and more specifically, in communication with the air chamber 21 of the burner 10. Furthermore, the air differential pressure sensor 32 is in communication with the housing through the pressure opening 17. The air differential pressure sensor 32 is also in communication with the controller 50. The air differential pressure sensor 32 detects the pressure differential between the air chamber 21 of the burner 10 and the housing 15, and sends a signal indicative of this difference to the controller 50. The temperature sensor T is also provided in the housing and is in communication with the microprocessor 50 to adjust the burner output. Knowledge of the differential air and fuel pressures allows the air / fuel ratio of the burner to be regulated exactly over the ignition range of the desired burner. From Figure 2, it is found that the differential air / fuel pressure ratio is not constant over the range of ignition proportions. Therefore, for accurate regulation, a proportional or linear control system is not suitable. To follow exactly the curves shown, a non-linear control system is required. It is important to detect the pressure in the housing or in the chamber 15 where the burner 10 is turned on, thereby taking into consideration the changes in the pressures of the chamber 15 when the flows to the burner are regulated. Housing pressure affects the stability of the burner flame, the flame output, and the air / fuel ratio. Although any suitable pressure sensor can be used, differential pressure transducers are preferably used. In the preferred embodiment of the present invention, a control valve 45 regulates the flow of fuel to the fuel chamber 22 of the burner 10. The valve 45 is in electrical communication with the controller 50. The air flow to the burner is regulated using a combustion blower, more preferably a fan 40 driven by variable speed drive. The fan 40 is in fluid communication, through suitable pipes (not shown) with the air chamber 21 of the burner 10. The drive 41 for the fan 40 is in electrical communication with the controller 50 as shown. The use of a variable speed drive fan with an acceleration and deceleration control that provides a superior balance of the air / fuel ratio and electrical savings during changes in the proportion of the burner combustion compared to a system where The air flow is modulated with a shock absorber and a drive device. The faster burner modulation without sacrificing the accuracy of air / fuel ratio control is achievable. Furthermore, the use of a variable speed motor to control the exit of the flame eliminates the disturbances of the flow produced by the damper, whereby the noise produced by the air flow in high combustion proportions is greatly reduced. During periods of low combustion proportions that are typical in most burner operation, the drive arrangement of the motor of the present invention is more energy efficient and quieter than a constant speed motor with a damper. In operation, the system monitors the differential air pressure between the burner air chamber 21 and the housing 15. The fuel flow is also observed by means of a measurement of the differential pressure between the burner fuel chamber 22 and the burner. housing 15. The signals indicative of these differential pressure measurements are sent to the controller 50, where they are compared with the experimental values or the curves supplied by the supplier (Figure 2) which are based on the burner's combustion rate. If the density of the air entering the combustion fan changes due to atmospheric pressure or temperature variations, the air differential pressure sensor detects the corresponding density in relation to the pressure variation and adjusts the fan output to compensate the change. A proper adjustment of the air / fuel ratio of the burner results in efficient burner operation with the lowest emissions. This also results in the length of the burner flame remaining short, which can be particularly advantageous in a drying system heated through the extraction which may require that the burner be in close proximity to the fan inlet. A long flame can damage the intake cone and fan wheel due to high temperature gradients if the flame hits the fan components. Another advantage of this system over the conventional mechanically controlled system is the ability to change the air / fuel ratio at any time or point of operation in a process. This can allow one oxidant to run at a rate during startup and another proportion during the current operating cycle. Mechanical air / fuel regulation systems can not easily or cost effectively accommodate changes during operation. Also, a change in the type of fuel can be carried out with established non-physical changes required for the burner. Example 1 In order to determine the optimum performance of a burner in terms of NOx, CO and CH4 emissions, a burner is started in the pilot mode and then the burner outlet is linearly raised from 0 to 100% and back to the pilot position by means of the control PLC. All signals are run in the PLC. The corresponding data is extracted from the PLC by means of a direct data exchange (DDE) linked to a personal computer in Microsoft EXCEL at a time interval of 1 second. An Enerac Portable FID combustion analyzer is used to generate the CH4 ppm signal. The output of the burner air temperature controller (TIC CV CV (%)), the reference point of the burner gas differential pressure (SP), the burner gas differential pressure (PV) process variable, the burner gas differential pressure controller output (%), the burner air differential pressure (SP) reference point, the burner air differential pressure (PV) process variable, the controller output of burner gas differential pressure (%) are recorded with the CO and N0X measurements using the same time sampling base and the corresponding graphs are plotted as shown in Figures 4, 5 and 6. The values of the proportion Gas / air pressure are calculated on the extended sheet in EXCEL. Figure 4 shows low NOx if the fuel / air pressure ratio stays close to 2.2. Figure 5 shows data using a burner having the instant control apparatus. It is noted that if the fuel / air pressure ratio remains close to 2.2, the unburned methane will be less than 10 ppm. Figure 6 shows that the CO is essentially zero ppm over the opening range of the total valve. Again, the fuel / air pressure ratio is close to 2.2 except in the small valve openings, typically less than 10%. Figure 7 shows the tracking of the current air pressure against the desired reference point over the range of the total valve. Figure 8 shows the tracking of the current gas pressure over the desired reference point for the range of the total valve. These data show that the control device tracks very well.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention

Claims

CLAIMS Having described the invention as above, the content of the following claims is claimed as property. 1. A control system for controlling the proportion of air with the fuel in the combustion of the burner in a combustion chamber, the burner has a fuel chamber and an air chamber, the control system comprising: means for detecting the differential fuel pressure for measuring the pressure differential between the fuel chamber and a combustion chamber and generating a first signal indicative of the measurement; means for detecting the differential pressure of the air for measuring the pressure differential between the air chamber and the combustion chamber and generating a second signal indicative of the measurement; means for controlling the flow of fuel to control the flow of fuel to the fuel chamber of the burner; means for controlling air flow to control the flow of air to the burner's air chamber; and control means responsively coupled to the fuel differential pressure detection means, to the air differential pressure detecting means and to the fuel and air flow control means to maintain the fuel and air ratio which are fed to the burner at a predetermined level. The control system according to claim 2, characterized in that the control means compare the first and second signals to the predetermined values. 3. The control system according to claim 1, characterized in that the predetermined values are not linear. The control system according to claim 1, characterized in that the means for controlling the air flow comprise a drive fan with variable speed. 5. The control system according to claim 4, characterized in that the variable speed drive comprises dynamic braking. The control system according to claim 4, characterized in that the fan comprises the acceleration and deceleration control. 7. A process to control the proportion of air and fuel in combustion in a burner in a combustion chamber, the burner has a combustion chamber, the burner has a fuel chamber and an air chamber, the process comprises: measuring the pressure differential between the fuel chamber and the combustion chamber and generate a first signal indicative of the measurement; measuring the pressure differential between the air chamber and the combustion chamber and generating a second signal indicative of the measurement; providing means of controlling the flow of fuel to control the flow of fuel to the burner's fuel chamber; providing means for controlling the air flow to control the flow of air to the burner's air chamber; and comparing the first and second signals with the predetermined values, and regulating the flow of air and fuel to the burner by means of controlling the flow of fuel and air in response to the comparison. 8. The process according to claim 7, characterized in that the air flow control means comprise an air blower fan. ---. £ ".. .._ .. variable speed. 9. The process according to claim 8, characterized in that the variable speed drive comprises a dynamic braking. The process according to claim 8, characterized in that the variable speed drive comprises controlling the acceleration and deceleration. 11. The process according to claim 7, characterized in that the first and second signals are compared with the non-linear predetermined values.